WASHINGTON, March 24 (Reuters) - The U.S. Navy said on Monday it was dismantling a temporary ice camp used to support submarine exercises off the coast of Alaska a week early after cracks appeared in the ice and made it too risky to use airfields to ferry people and supplies.

"The rapidly changing conditions of the ice, along with extremely low temperatures and poor visibility have hampered helicopter operations and made sustaining the runway and camp too risky," the Navy division in charge of submarine forces said in a statement. It said all personnel at the camp were safe.

Navy officials said the instability of the ice underscored growing concern about the changing Arctic environment.

A breakdown of Camp Nautilus began on Sunday, the Navy said in the statement. The camp was due to continue supporting submarine exercises below the surface through March 30.

Substantial shifts in wind direction created instability in the Arctic Ocean's wind-driven ice floes, resulting in fractures in ice near the camp that prevented the use of several airfields, the Navy said. Small fixed-wing planes or helicopters are typically used to transport personnel, visitors, and supplies to the camp.

This is the first time since 2011 that the U.S. Navy has built a temporary base camp on the ice far north of Prudhoe Bay to support submarine exercises. It carried out a similar exercise in 2012 without a camp.

The camp is essentially a small village with housing, a mess tent and buildings where scientific and military exercises are coordinated and conducted.

Officials at the Navy's Arctic Submarine Laboratory, which plans the exercises, had no immediate comment.

Ice Exercise 2014 began on March 19 as part of the Navy's training in the Arctic Ocean, which is a route for submarines between the Atlantic and Pacific Oceans. The camp opened on March 17, before the submarines arrived.

The Navy released a plan in February for expanding its presence in the Arctic beginning around 2020 after analysis showed that seasonal ice was disappearing faster than had been anticipated as recently as three years ago.

In 2011, the Navy noted a reduction in the thickness of the ice in the area where the base camp had been located in the past. Thickness of the ice was a concern this year in selection of a camp site.

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For an interview with the Navy's top oceanographer, click on http://www.reuters.com/article/2014/03/01/us-usa-arctic-navy-idUSBREA1Q2DU20140301

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The Navy has said this year's exercise included the USS New Mexico, a Virginia-class attack submarine built by Huntington Ingalls Industries and General Dynamics Corp, and USS Hampton, a Los Angeles-class attack submarine.

Virginia-class submarines are next-generation attack submarines that can also be used in shallow-water, while the Los Angeles-class ships are better suited for deep water operations. Both carry Tomahawk cruise missiles.

Submarines have conducted under-ice operations in the Arctic region for more than 50 years to support inter-fleet transit, training, engagements with allies and military operations.

The Navy has conducted 120 Arctic exercises, many together with British submarines, since the USS Nautilus submarine made its first transit of the region in 1958. (Reporting by Andrea Shalal; Editing by Ros Krasny and Sandra Maler)